It is often required to obtain an electrical signal representing the angular position of a rotatable element. In automotive applications the element may be a fuel level sender controlled by a float, a feedback for an instrument gauge, or a manually positioned knob for example. It is also desirable that the position information be in digital form for easy communication over a bus to another part of the vehicle.
For some applications it is already known to use rotary optical sensors, code wheels with inductive pickups, and rotary potentiometers. The most common variety of fuel level sensors consists of a float which is attached to the end of a pivot arm. A potentiometric or resistive divider device is operated by the pivot arm so that the value of resistance or the ratio of resistances corresponds to the fuel level. Where a wiper sweeps across a wire winding or across resistive inks, a fairly high amount of current flowing through the sensor is required to keep the contact from fouling over time. These higher currents require that electronic driving circuits use larger wattage resistive loads to operate the sender which can represent a significant source of heat generation in the instrument cluster. Whereas air core gauges utilize the analog sender signal, newer display technology often requires digital data, requiring analog to digital conversion. An additional concern with such senders is the presence of corrosive materials in fuel including alcohol and water, and sometimes salt water.
Lineal capacitive sensors using digitally excited electrodes and relatively movable sensing elements have been used with a phase detecting circuit to measure position along a straight line. It is desirable to employ the principles of such sensor in a capacitive rotary encoder suitable for liquid level and other measurements.